1. Technical Field
This invention relates to polyacetal compositions containing certain hindered amine light stabilizers ("HALS").
Polyacetal (also commonly referred to as polyoxymethylene) compositions are generally understood to include compositions based on homopolymers of formaldehyde, the terminal groups of which are end-capped by esterification or etherification, as well as copolymers of formaldehyde or of cyclic oligomers of formaldehyde and other monomers that yield oxyalkylene groups with at least two adjacent carbon atoms in the main chain, the terminal groups of which copolymers can be hydroxy terminated or can be end-capped by esterification or etherification. The proportion of the comonomers can be up to 20 percent weight.
Compositions based on polyacetals of relatively high molecular weight (i.e., 10,000 to 100,000) are useful in preparing semi-finished and finished articles by any of the techniques commonly used with thermoplastic materials, e.g., compression molding, injection molding, extrusion, blow molding, rotational molding, melt spinning, stamping, thermoforming and the like. Finished articles made from such compositions possess desirable physical properties, including high stiffness, strength, low coefficient of friction, and good solvent resistance.
In some applications, polyacetal compositions are exposed to ultraviolet ("UV") light for long periods of time. It is desired that under such circumstances, the polyacetal composition remain relatively stable. To impart or improve UV light stability to a polyacetal composition, HALS may be added to the polyacetal compositions. However, it is not uncommon that the addition of a HALS to a polyacetal composition can adversely affect other properties of the polyacetal composition, especially its melt processing thermal stability.
It has been found, in the present invention, that the inclusion of certain HALS into a polyacetal composition results in a polyacetal composition having good stability upon exposure to UV light, as measured by weight loss upon exposure to UV light for a set period of time. Further, it has been found that the inclusion of these certain HALS into a polyacetal composition does not significantly adversely affect other properties (in particular, melt processing thermal stability (as measured by a thermally evolved formaldehyde test)), of the polyacetal composition.
The present invention more specifically relates to polyacetal homopolymer compositions containing at least one HALS that contains a hindered phenol group and that further contains a piperidine ring or an oxo-piperazinyl ring. Both the piperidine ring and the oxo-piperazinyl ring contain a hindered amine, at the N.sup.4 position in the ring, that is of tertiary functionality. These types of HALS are referred to herein as either "Type I HALS" or "Type I (component b) HALS".
The present invention also relates to polyacetal compositions containing at least one HALS as described in the preceding paragraph (i.e., Type I HALS) and additionally, at least one HALS that has an s-triazine ring and a piperidine ring or an oxo-piperazinyl ring. Both the piperidine ring and the oxo-piperazinyl ring contain a hindered amine, at the N.sup.4 position in the ring, which may be of secondary or tertiary functionality. This latter type of HALS is referred to herein as either "Type II HALS" or "Type II (component c) HALS".
The compositions of the present invention are useful wherever it is desired to use a polyacetal composition having enhanced UV and thermal stability, both during and after melt processing. Articles prepared from the compositions of the present invention can be used, for example, in automotive or irrigation applications.
2. Background Art
U.S. Pat. No. 4,863,981 discloses a polymer stabilizer mixture containing (a) a polyalkyl piperidine hindered amine light stabilizer component wherein the hindered amine in the piperidine ring is of tertiary functionality and (b) a polyalkylpiperidine hindered amine light stabilizer component which incorporates an s-triazine residue. The component (a) HALS does not contain a hindered phenol group.
U.S. Pat. No. 4,446,263 discloses a polyacetal copolymer stabilizer mixture of (a) 1,6-hexamethylene bis(3,5-di-tert-butyl)-4-hydroxyhydrocinnamate, (b) 2,4,6-triamino-sym-triazine, (c) bis (1,2,2,6,6-pentamethyl-4- piperidinyl) [3,5-bis(1,1-dimethylethyl-4-hydroxy- phenyl) methyl] butylpropanedioate (sold commercially as Tinuvin.RTM. 144/Ciba Geigy), and (d) 2-hydroxy-4-n-octoxy-benzophenone (sold commercially as Cyasorb.RTM. UV 531). In the stabilizer mixture, component (a) serves as an antioxidant, component (b) serves as an acid scavenger, component (c) serves as a free radical scavenger, and component (d) serves as an acid scavenger. The stabilizer mixture is taught to be useful specifically in polyacetal copolymer compositions. There is no teaching that said stabilizer mixture would be useful in polyacetal homopolymer compositions. In fact, as is shown in the Examples below, the component (b) of said stabilizer package severely degrades the melt processing thermal stability of polyacetal homopolymer compositions and as such, would not be acceptable for use therein.